The present invention relates to wireless terminals and operation thereof, and more particularly, to transmit power amplifiers and operations thereof.
Transmitters in wireless terminals, such cellular handsets, often are designed based on an assumption that the power amplifier of the transmitter drives an approximately 50 ohm antenna impedance. However, antennas for such devices are increasingly being adapted for use over multiple bands and being integrated into the handset housing, which may cause significant divergence of antenna impedance from the 50 ohm assumption under a wide array of operating conditions. For example, in some newer antenna designs, voltage standing wave ratio (VSWR) may be nominally better than 3:1, but can go to 6:1 or worse when a user simply places a hand or other object near the antenna. The amount of power that can be transmitted through an antenna is affected by impedance matching between the amplifier and the antenna impedance, and poor impedance matching between the transmit power amplifier and the antenna can negatively affect the ability to maintain a call.
Some wireless terminals use closed-loop techniques to control a bias of the transmit power amplifier. Some conventional terminals directly sense the output of the power amplifier using, for example, a diode or other detector coupled to the radio frequency (RF) output of the power amplifier, and adjust the bias voltage and/or the signal input to the power amplifier responsive to the detected output. Such techniques, while effective, may require relatively complex and costly circuitry. Other closed loop techniques involve regulating power supply current provided to the amplifier. For example, a cellular handset may include circuitry that measures the power supply current to the power amplifier and adjusts the bias voltage applied to the power amplifier to maintain a substantially constant power supply current to the power amplifier. Such a technique can be relatively easily and inexpensively implemented, and may work well when the antenna impedance is relatively constant.